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Hi. Thanks for using JA! I will try and help you out.
Code 55 indicates that the ECM does not have any diagnostic codes.
As I'm sure you know, 30 psi is too low for your engine. Since you have inspected the turbos and they are OK, the next step would be to pressure check the air to air cooler. I have seen them crack and cause a boost leak and in turn causing low power.
If the air coloer is good and does not have any cracks, the next thing I would want to do is check the fuel pressure of the engine. It is possible that the fuel system pressure is low and in turn will lower the fuel delivery to the engine, and then lower the boost pressure and power of the engine.
Please let me know if you need more help.